1. Technical Field
Over the last few years it has become increasingly common to connect personal computers or workstations together into networks so that different machines can communicate with one another. A typical approach has been the introduction of local area networks (LANs) that provide file transfer and other facilities between computers. Other types of link, for example Integrated Services Digital Network (ISDN), are also known.
2. Description of the Prior Art
Modern networks can support the development of facilities such as video conferencing, in which a video image of a user captured at one terminal appears on the screen at a second terminal, and vice versa. A typical video conferencing terminal is a personal computer equipped with a video camera and an adapter card that allows video images to be displayed within a window on the screen of the personal computer. These adapter cards were often first designed for displaying video stored on compact disks, but have now been incorporated into video conferencing systems. The use of such adapter cards in multimedia applications is described in xe2x80x9cMultimediaxe2x80x9d, p112-p123, PC Magazine, Mar. 31, 1992, and xe2x80x9cBack to Realityxe2x80x9d, p212-p223, Personal Computer Magazine, June 1992.
The very high data rates required to store, display or transmit video signals have compelled various forms of data compression to be used in order to reduce the amount of processing required. Typically this compression is performed by special purpose hardware on the adapter capable of operating at the required high speed. There is no single standard form of data compression for video signals, so that different hardware adapters each use their own particular compression algorithms: a result of this video compact disks are specific to a particular hardware adapter. This leads to a serious problem in video conferencing over heterogeneous networks, in which the workstations in the network are equipped with a variety of different types of hardware adapter. It is generally impossible for a compressed video signal produced by one type of adapter card and transmitted over the network to be decompressed and displayed at another workstation equipped with a different type of adapter card.
Accordingly, the invention provides a computer workstation including means for generating a video signal, means for transmitting the video signal over a network to one or more other computer workstations, and further including:
a first video subsystem comprising means for processing the generated video signal into a first predetermined compressed format and for passing it to the video transmission means;
a second video subsystem comprising means for processing the generated video signal into a second predetermined format and for passing it to the video transmission means;
and means for selecting whether to transmit a video signal in the first or second predetermined format to any particular node in the network.
Thus the computer workstation is effectively equipped with two parallel video subsystems that can produce different video outputs for transmission over the network (based on the same basic video signal). The first video subsystem is typically hardware based, and produces a compressed signal that is specific to that hardware unit. This compressed format can then be transmitted over the network with relatively low bandwidth requirements, but it is only possible for terminals equipped with compatible hardware to display this signal. Thus the computer workstation also includes a second video subsystem, which is designed to produce a much simpler video signal. This subsystem is based on software, with no special purpose hardware. The signal produced by this subsystem will be either uncompressed, or use a relatively simple compression algorithm that can easily be implemented in software. This signal can then be transmitted to those displays at terminals that do not have hardware compatible with that at the transmitting workstation. The lesser degree of compression for the signal in the second predetermined format will of course result in a lesser frame rate, but nevertheless it has been found that an acceptable video signal can be transmitted. The means for selecting the video format to use can be a simple switch based on stored information: e.g., for a given terminal use data in a given format. Alternatively, in a dynamically configured network, the means for selecting can provide a method whereby to terminals can negotiate to select which video format is most appropriate to the transmission of a video signal between them.
Correspondingly, the invention also provides a computer workstation including means for receiving an incoming video signal from a network, means for displaying the video signal, and further including:
a first video subsystem comprising means for processing a video signal received in a first predetermined compressed format and for passing it to the video display means;
a second video subsystem comprising means for processing a video signal received in a second predetermined format and for passing it to the video display means;
and wherein the receiving means includes means for forwarding the incoming video signal to the first or second video system.
Again, the computer workstation has first and second video subsystems, one of which will typically be based on specific hardware, such as a video adapter card, and one of which is software based. If the workstation is connected to a transmitting terminal with compatible hardware, then the workstation arranges for the video signal to be sent in the hardware-specific format for processing by the first video subsystem containing the appropriate hardware. However, if the receiving and transmitting workstations do not have compatible hardware, the data is sent in a second predetermined format. This format is much simpler, and the second video subsystem can be used to display this signal without the need for any specific hardware in the workstation. Again, the simpler video format used by the software unit will be less compressed than the hardware signal, and so either require greater bandwidth on the network, or else result in a lower frame rate.
In a preferred embodiment, the second predetermined format comprises representing the video signal as a series of bit maps, and the second video subsystem includes means for calling operating system routines to pass the bit maps to the video display means. Thus the second video subsystem requires no special hardware, but simply exploits the facilities already provided by the operating system of the workstation. The use of bit maps for the second predetermined format is a convenient and simple choice, but any format that allows easy compression/decompression by software could be used.
Of course, in general workstations will contain both the transmission and reception facilities described above, to allow them to fully participate in two-way video conferencing. Thus they will be capable both of transmitting a video signal in two (or more) different formats and of displaying an incoming video signal in two (or more) different formats. One of these formats will be a very simple software-processable format, such as a simple bit map, which will therefore act as a sort of lowest common denominator across the network to allow all the terminals to communicate with each other.
Preferably the video transmission means or the video receiving means is unaware whether said video signal is in the first or second predetermined format. In general the workstation will be running an application, which is responsible for transmitting or receiving the video signal. The fact that more than one video format is supported is therefore transparent to the application, which simply knows that a connection has been established with another terminal. This allows applications to be developed without having to know details about the video capture of display systems that are present at any given terminal.
The invention also provides a method for displaying a video signal received from a network at a computer workstation including video display means comprising:
determining whether to forward the video signal to a first or second video subsystem, and forwarding a video signal in a first predetermined format to the first video subsystem, and forwarding a video signal in a second predetermined format to a second video subsystem; processing the video signal at said first or second video subsystem; and forwarding the video signal from said first or second video subsystem to the video display means.
The invention further provides a method for generating a video signal at a computer workstation, said workstation including means for transmitting the video signal over a network to one or more other computer workstations, including the steps of:
converting the video signal into first or second predetermined format;
processing a video signal in the first predetermined format at a first video subsystem; processing a video signal in the second predetermined format at a second video subsystem;
and passing the video signal from the first or second video subsystem to the video transmitting means.